it sounds like other plugin is not using wordpress meta well - else data would show.

users plugin does try to dig down and into arrays to extract and make the data as meaningful as possible. It sounds like it is not succeeding here or perhaps there is something odd about how other plugin stores data.

1) what is other plugin ?
2) did you run 'find fields' with existing sample data in DB?. Were fields found ? what is special about fields ?

LearnDash is a paid e-learning plugin.
BadgeOS is a free open source award system plugin, with a very complete API.
Both are totally awesome and can work together : the user can do a quiz from LearnDash, and be reward by a badge if he succeed.
They have a great support if you want to contact them (I already spoke to your plugin to the LearnDash developper and he was quite interested).

So, what I'm trying to do is to display withn the user list the badges images of the user (and if possible, this img are link to the individual badge pages) and the course progression information from learndash.

I think I that the strings in amr users :

_badgeos_achievements
_sfwd-course_progress

But the first display "Array" and the other "Array, Array, Array, Array", as you cans see on this screenshot.